The invention relates to scattered-light smoke detectors.
Smoke detectors involving sensing of optical properties of combustion aerosols are in common use, especially as based on scattered-light principles. Such detectors are suited for early-warning fire detection, for timely fire-fighting intervention.
For reliability of response, the sensitivity of fire detectors must lie within a certain tolerance interval, typically as prescribed by technical standards or regulations. Accordingly, it is important to provide means for adjusting the sensitivity of scattered-light smoke detectors.
A scattered-light smoke detector includes a light source, typically for emitting light pulses into a spatial region of the detector accessible to combustion aerosols. In the spatial region, light from the source is scattered by the combustion aerosols. Included further is a light sensor which is designed and disposed to detect light from a spatial subregion. This subregion may be called measurement volume.
In the interest of preventing light from reaching the sensor in the absence of smoke, elaborate light traps or baffles are included for shielding the sensor against spurious influences, mainly from dust particles on surfaces of the spatial region. But even in a pristine detector, no matter how elaborately designed, a small amount of light will be reflected from these surfaces, resulting in a base-level signal.
An electrical signal produced by the scattered light in analyzed in an evaluation circuit, and, if a sensor output signal exceeds a predetermined threshold, an alarm signal is triggered. Scattered-light smoke detectors of this type are described in numerous patent documents, e.g., GB-A-2,251,067 and DE-G-8,524,914.
Typically, the sensitivity of scattered-light smoke detectors is set in the course of manufacture. According to a frequently employed method, scattered-light smoke detectors are placed in a chamber or passage which can be filled with a test aerosol having known composition and concentration. Upon adjustment of this concentration to an alarm concentration, the sensitivity of the detector is set by appropriate adjustment of the alarm threshold, for production of an alarm signal at predetermined smoke concentration.
This method of adjustment has significant drawbacks, impeding manufacture. For one thing, it is difficult to produce a calibration aerosol with controlled concentration. For another, the method is time consuming. In fact, the adjustment step including production and control of the calibration aerosol is determinative of the rate of assembly. To achieve assembly rates as are expected in modern assembly line production, several smoke calibration installations have to be operated in parallel, with attendant high requirements of uniformity of control.
In an alternative method, without use of smoke for calibration, the above-mentioned base-level reflection is used as a reference. From a reference signal produced by the base-level reflection, a suitably higher signal value is chosen as the alarm threshold value. While this method of calibration is considerably faster, it has a decided drawback in requiring a high degree of constancy of the base-level reflection, i.e., of the physical properties of delimiting surfaces. The optical trap must be built to such high standards that the rejection rate and thus the manufacturing costs are high. This is one of the reasons why most detector manufacture still involves calibration with smoke, in spite of greater complexity.
Mainly, however, use of the base-level reflection as calibration reference has the drawback of not offering a true simulation of an aerosol, and thus of not representing a physically adequate alternative to calibration with smoke of known concentration.
Scattering of light by smoke particles is a volume effect, i.e., the scattered light received by the sensor is the sum total of many individual scattering processes in the measurement volume. By contrast, base-level reflection is a surface effect. Light reaching the sensor originates on interior detector surfaces and varies depending on the properties of these surfaces. There is no simulation of the physical effect for which the detector is designed, and detectors "calibrated" by this method cannot be expected to have uniform sensitivity to smoke. The ability of a detector to sense the presence of light-scattering particles in the measurement volume remains untested.
Other methods are known which involve insertion of a test object into the measurement volume of a scattered-light smoke detector. Such a method is described in Japanese Patent Document JP-53-99899, disclosing insertion of a needle shaped object into the measurement volume from the outside for testing of the detector.
According to the disclosure of British Patent Document GB-1,079,929, the presence of smoke is simulated by insertion of a flag into the measurement volume.
U.S. Pat. No. 3,585,621 discloses a functional test, involving placement of a calibration object opposite the light source, having a reflectivity corresponding to the scattering by smoke of a given density. Here again, simulation is not realistic, as light is merely reflected from the surface of the object rather than scattered by many particles as in the case of smoke.
U.S. Pat. No. 4,099,178 discloses a test setup which provides for primary light from the light source to pass through a small opening in the light trap directly to the sensor. No realistic simulation of scattering by a plurality of particles is achieved, and the technique is only conditionally suited for functional testing of a detector, as the intensity of light reaching the sensor is larger by magnitudes as compared with scattered radiation from combustion aerosols.
Neither of these calibration techniques is sufficiently accurate to replace calibration with an aerosol.